PROJECT SUMMARY Immunotherapy is transforming decades of clinical practice in cancer care, but it also comes with new questions about understanding mechanisms of action contributing to both antitumor activity and potential associated toxicity. Most importantly, identifying why only a fraction of patients derives clinical benefit is at the forefront of future developments, with the ever-elusive validation of useful clinical biomarkers as the ultimate goal. As a consortium of immunologists, technologists, clinicians, computational biologists, data specialists, and biostatisticians, the Mount Sinai CIMAC (MS-CIMAC) is uniquely positioned to generate immune profiling datasets at an unprecedented level of granularity to identify biomarker signatures of disease course and response to immunotherapy in cancer patients. MS-CIMAC will take full advantage of the smart cancer immunotherapy trial designs championed by the NCI that mandate collection of baseline and on-treatment biospecimens. Key strengths of MS-CIMAC are expected to be in unique high-dimensional yet sample-sparing approaches, including CyTOF mass cytometry, seromics, microbiome, and multiplex chromogen IHC, thanks to the implementation of cutting-edge yet validated protocols and analysis pipelines. Through a comprehensive array of assays and analytical tools that bridge innovation and standardization, MS-CIMAC intends to pursue the following three aims: a) help characterize immunocompetence at baseline and assess global immune changes during treatment, b) drill down the specificity and quality of immune responses for mechanistic evaluation of drugs, and c) automate, optimize, and integrate analyses of resulting datasets to facilitate sharing, and to ultimately discover composite immune biomarkers that will impact clinical cancer care.